Rhythmic changes in velocity, volume, and flow of blood in the optic nerve head tissue

Using laser Doppler flowmetry, slow variations in velocity, volume, and flux of red blood cells in the optic nerve head (ONH), choroid, and retina of the anesthetized minipig have been demonstrated. The variations of velocity and volume were highly regular and vigorous in the ONH and had frequencies ranging from 2.5 to 4.5 cycles/min. The flux variations were smaller or absent due to a phase shift of approximately 180 degrees between the volume and velocity changes. The volume fluctuations were synchronized to those of the PO2 measured in the vitreous, at approximately 50 microns from the surface of the ONH tissue. The fluctuations were less regular in the choroidal and the retinal vessels and their frequencies were higher than those in the ONH. The lack of correlation between the fluctuations in the ONH and those in the retinal and choroidal vessels points toward a local mechanism. The changes in blood volume in the ONH, the phase shift between volume and velocity changes, and the predominance of venules at the surface of the ONH are three factors suggesting that this mechanism involves a change in the diameter of the venules rather than in the arterioles.